We fabricate and analyse Cu(In,Ga)Se-2-based solar cells which have a graded band gap by an increased Ga content towards the Mo back contact. The open circuit voltage and the short circuit current strongly improve with the band gap grading. Electronic device analysis reveals that the open circuit voltage increase is directly related to the increased band gap energy at the back surface. We interpret the obtained results to a large extent as reduced back contact recombination by the introduction of an increased band gap close to the back contact.